In various mechanisms for transmitting a rotational torque, the end portions of two rotating shafts are coupled by means of a coupling. For example, an output rotating shaft of a motor and an input rotating shaft of a pump are coupled to each other by means of the coupling. In this case, it takes a considerable amount of labor to carefully install the motor and the pump so that the output shaft of the motor and the input shaft of the pump may satisfactorily align. In addition, even if they are installed by paying a full attention to assuring such an alignment, some eccentricity or deviation of angle remains between both the shafts and, further, since vibrations occur in the motor and the pump during the operation, a flexible coupling has conventionally been used as the coupling in order to absorb them by the coupling portion. As the conventional coupling, ones using a flexible member such as a spring, wire or the like may be enumerated by way of example. However, with those flexible couplings, when the eccentricity, deviation of angle and axial movement occur, since the absorption of a relative displacement between the two rotating shafts is assigned only to a deformation of the flexible member, in order to enlarge an extent to which the eccentricity, deviation of angle and axial movement are allowed, the deformation of the flexible member must be large. In consequence, a soft member must be used as the flexible member and, especially when a large torque is transmitted, the torque cannot excellently be transmitted during a transition period when an equilibrium state is being changed. In addition, a considerable amount of energy is consumed to deform the flexible member, and an efficiency of transmitting the torque is lowered.
In addition, an Oldham coupling has been used as a coupling which copes with the above-described eccentricity, deviation of angle and axial movement. However, for a conventionally used Oldham coupling, a member for transmitting the torque is disposed between a member at the side of the driving shaft and a member at the side of the driven shaft so that the torque transmitting member and the member at the side of the driving shaft may relatively slide in a first direction within a plane intersecting at a right angle with the rotational axis while the torque transmitting member and the member at the side of the driven shaft may slide in a second direction intersecting at a right angle with the above-described first direction and being within a plane intersecting at a right angle with the rotational axis, these slidings being each achieved by engaging a single slide member within a single channel. As a result, the conventional Oldham coupling has been used as one in which the area of the slide portion for transmitting the torque is small and a relatively small torque is to be transmitted. Further, for the conventionally used Oldham coupling, since the above-described channel and the slide member are each made of metal, it is necessary to arrange so that a lubricating oil exists therebetween, which in turn causes a troublesome maintenance. In addition, with the conventional Oldham coupling, when the equilibrium state is abruptly changed as when a load is abruptly changed, an excessive stress has been applied thereto and, further, the vibration is easy to be transmitted between the driving shaft side and the driven shaft side. In order to overcome such drawbacks, the Oldham coupling and the above-described flexible coupling have often been used by connecting, which, however, undesirably causes enlargement of the size as well as increase of the cost.